This invention relates to fuses in general and in particular to an electric time delay fuse. A time-delay fuse is a type of fuse that has a built in delay that allows temporary and harmless inrush currents to pass without opening, yet is designed to open on sustained overloads and short circuits.
Underwriter's Laboratories has developed basic physical specifications and electrical performance requirements for fuses with voltage ratings of 600 volts or less. These requirements are known as UL Standards. If a type of fuse meets the requirements of a standard, it will be placed in that UL Class. Typical UL Classes are K-1,K-5, RK-1, RK-5, G, L, H, T, CC, and J.
Those UL classes which are labeled as "current limiting", have physical rejection features and are not interchangeable with other classes. The UL specification for Class J fuses having time delay requires the fuse to be fast clearing and tolerate a 500% overload for 10 seconds. In addition, in order for a fuse to meet the Class J requirements, it must meet the voltage, current characteristics, and physical size requirements of Underwriter's Laboratories. Thus the time-delay element and the short circuit element must be small and compact. Further, it is necessary to have a fuse which has a high interrupting rating and is fast acting during short circuit interruptions.
The objective of the class J time delay fuse for motor protection is twofold: a) to provide faster interruption during short circuit than other classes of motor protection fuses; and b) to withstand normal motor start up without nuisance opening. To achieve both of these requirements, designers must solve the problem of the common fuse link behavior. That is, an inability of a fast clearing fuse to tolerate 500% overload for 10 seconds.
Many fuse designs have only one element. This element typically consists of a single strip of material called a fuse element. During a short circuit condition, the fuse element is violently heated to a point where a section of the strip melts and disintegrates. It is the disintegration of the fuse element which interrupts the electric circuit.
Single-element fuses are not satisfactory for situations where momentary overload conditions are common. In order for the fuse to clear quickly, the fuse elements are constructed to melt immediately upon high current events. Although this is desirable for short circuit conditions, a modest overload over a short period of time often causes the fuse element to melt and interrupt the circuit.
To overcome this problem, some prior designs employed a thicker fuse element. Although this eliminated nuisance overload interruptions, it also made the fuse element more resistant to fast interruption during short circuit events, thereby increasing the damage to expensive equipment. This characteristic makes single-element designs undesirable for motor starting protection where momentary overloads are typical during the starting process.
Many of the problems of the motor starter fuse protection are solved by employing a dual-element fuse design. The dual-element fuse contains two distinctly separate elements which are electrically connected in series. The first element, called the overload element, will interrupt the circuit when the current is five times higher than the fuse rating for more than 10 seconds. The second element is called the short circuit element. It interrupts the circuit during short circuit events.
The overload element normally consists of a trigger mechanism. Although the trigger mechanism is adequate for overload situations, it does not clear quickly during a short circuit event--a requirement for good fuse performance. In order to have a fast clearing fuse, designers are encouraged to employ fast acting fuse elements in their designs. Thus the problem of nuisance interruption due to overload conditions still exists. However, the use of dual-element fuse obviates the designers dilemma of striking a good balance between overload and short circuit performance. Instead, fuse element designers may concentrate on methods to ensure fast clearing without the need to oversize the fuse element to survive an overload condition.
For short circuit interruption, some prior art devices relied on a trigger operation inside a loose sand matrix. This design was desirable because a loose sand filler is less expensive and easier to manufacture. However, the performance of this combination proved inadequate.
A second solution uses a fuse element surrounded by a better thermoconductive filler than loose sand. This alternate filler is called "stone sand," although there are other combinations of solid filler material besides stone sand. In all cases, the advantage of the solid filler is the increased thermoconductivity of the solid media surrounding the fuse element. By transferring heat to the solid filler, the fuse element is able to pass a 500% overload current longer before melting.
Although useful, the solid filler method does have drawbacks. The process of filling and solidification around the weak spots is difficult and expensive. Further, unless the filling process is taken with care, bubbles or gaps may appear near the fuse elements, thereby degrading performance.
Loose sand fillers are easier to place around fuse elements than solid fillers. However, because of the inherent gaps in the loose sand filler surrounding the fuse element, the loose sand filler cannot take absorb the heat generated within the fuse element during an overload condition.
A third solution to nuisance interruptions in fuse elements is the inclusion of low resistance weak spots in the fuse element. The heat generated by the short circuit event is concentrated at the weak spot, causing the fuse element to melt at that point. The dimension of the weak spot is chosen to provide fast clearing short circuit performance. The larger segments of the fuse element are available to absorb the heat generated during an overload condition. This means that the fuse element is, in effect, oversized for overload conditions while maintaining sort circuit performance. This method makes the use of expensive solid fillers unnecessary for some, but not all, applications.
In prior art designs, the weak spot is produced by a conventional stamping process. This technique relies on the widely accepted maxim that the minimum size of the weak spot is directly related to the minimum size of the punch used to produce the weak spot dimensions. Commonly, the limit for weak spot effective length is more than 0.018 inches. This length limit of the weak spot is proportional to the weak spot electrical resistance for a given cross section of the fuse element and is expressed by the following formula: ##EQU1## Where: R.sub.W.S. =Resistance of the weak spot.
p=Resistivity of the material of the fuse element. PA0 A.sub.W.S. =Cross-sectional area of the weak spot. PA0 L.sub.W.S. =Effective length of the weak spot. PA0 1. The distance between the weak spots must be greater than two arc lengths. One arc length being the maximum distance which a given fuse element will allow conduction of electricity through the arc generated during a short circuit event as the fuse element melts. Two arcs sufficiently close to one another will continue to conduct electricity. Therefore, it is important that a series of arcs not form, and allow the circuit to remain unbroken during a short circuit event; PA0 2. The total length of the fuse element which is normally set by the size limitation of the fuse; and PA0 3. The total resistance of the fuse element, which, if exceeded, may cause a reduction of the carrying capacity of the fuse or cause unacceptably high temperature during the 110% carrying test. PA0 V=Voltage. Thus, in order to satisfy the UL Class J requirement of 600 volts (rms), the number of weak spots must exceed six. It is the combination of the number of weak spots in excess of six, the shorter effective length of each weak spot, and bending of the fuse element to allow a longer fuse element within a tube that the fuse to perform up to UL Class J standards.
The value of p is constant for a given material, such as silver. The value of A is set by the limits for the I.sup.2 t values for Class J fuses and the stamping process limitations. It has been known that I.sup.2 t and IPEAK values can be controlled by the size of the cross sectional area of the weak spot.
For example, the cross sectional area A.sub.W.S. for the silver fuse element used in Class J fuses rated at 30 ampere must be smaller than 200 square mils. Thus, the cross sectional area A.sub.W.S. has to be smaller than some critical maximum in order to meet the requirement of UL for Class J fuses. Thus p and A.sub.W.S. may be taken as constants, which means that R.sub.W.S. must be proportional to L.sub.W.S.. In order to employ a loose sand filler and still withstand a 500% overload for more than 10 seconds required of a Class J fuse, the resistance of the weak spot must be reduced further than in prior art designs.
It is commonly known that weak spots in series can be utilized to improve performance of the fuse at a higher rated voltage. However, the maximum number of serial weak spots which may be placed along the fuse element is limited by several factors:
There is therefore a need in the art for a Class J rated fuse which can utilize a loose sand filler around the fuse element.